Description of the Related Art
A conventional waveguide type optical device comprises a LiNbO.sub.3 substrate of an electro-optical effect which is cut in Z axis, optical waveguides formed by thermally diffusing Ti into the substrate, a buffer layer of a SiO.sub.2 film covering a surface plane of the substrate, and electrodes provided via the buffer layer on the optical waveguides.
In operation, an input light is supplied to a light input port to be introduced into an optical waveguide, and the input light is divided into two lights to be propagated through the optical waveguides. At this time, modulating voltages are applied to the electrodes, so that refractive indexes of the optical waveguides are changed due to the electro-optical effect.
Consequently, a phase difference occurs between the two lights propagating through the optical waveguides, and the two lights having the phase difference are combined to be supplied to a light output port. Thus, an output light which is modulated in intensity dependent on the modulating voltage is obtained at the light output port.
In this waveguide type optical device, however, there is a disadvantage in that a predetermined modulation characteristic (extinction light ratio) is difficult to obtain, because a TE (transverse electric) mode light and a TM (transverse magnetic) mode light tend to copropagate through the optical waveguides. In more detail, a TE mode light having a polarization parallel to the surface plane of the substrate is introduced into the optical waveguide along with a TM mode light having a polarization orthogonal to the surface plane thereof, when a polarization characteristic of a polarization maintaining optical fiber coupled to the light input port is low, or the angle adjustment of the optical fiber and the optical waveguide is not appropriate at the light input port. As known in the art, modulating voltages are different between the TE and TM mode lights to provide a predetermined phase difference. As a result, the above described disadvantage is observed in the conventional waveguide type optical device.
In order to overcome the disadvantage, first and second structures are proposed in the conventional waveguide type optical device.
The first structure is to provide a SiO.sub.2 thin film having a refractive index lower than that of the optical waveguide in the vicinity of the light input port to be positioned on the optical waveguide, and a Si thin film having a refractive index higher than that of the optical waveguide on the SiO.sub.2 thin film.
In this first structure, the thickness of the SiO.sub.2 and Si thin films is adjusted to apply coupling obtained by phase matching between the Si thin film and the optical waveguide only to the TE mode light.
The second structure is to provide a tiny polarizer between the optical fiber and the light input port.
In this second structure, the TE mode light is removed by the tiny polarizer, while the TM mode light is transmitted thorough the polarizer to be introduced into the optical waveguide.
In the first structure, however, there is a disadvantage in that the fabrication of the waveguide type optical device is difficult, because the thickness of each thin film must be precisely controlled. In addition, a thin film thickness which is determined under the condition where only a TE mode light is attenuated is not a practical thickness which is applied to an actual waveguide type optical device.
The second structure has also a disadvantage in that a fabricating process becomes complicated, because the tiny polarizer which is very small in size is fixed at a predetermined position by adhesive, thereby increasing positions on which members are fixed by adhesive. In addition, the number of optically coupling connections is increased to lower the reliability.